Structural controls and paragenesis of the Big River South

Shand, Fraser Charles Grant, 1992 (Fraser)

The Big River South – St George deposits are hosted within Sunderlands lode series, in the south east of the Reefton Goldfield. Macroscopic folds flank both deposits. Three bedding parallel faults host the auriferous quartz reefs at each deposit, with the central reef being the principal ore zone. These reefs are offset by later E-W trending faults. There are also several auriferous quartz stockworks surrounding the mines. The primary ore type comprises large recrystallised quartz veins with sulphitic stylolites. These veins host visible gold. Lower concentrations of gold have been identified within mineralised greywacke adjacent to the lode, whilst a lower permeability may have prevented mineralisation within argillitic horizons. Fault valve behaviour along with a variable fluid composition has resulted in a complicated paragenetic sequence of mineralisation. However, there appear to be three dominant ore-forming stages. Initially Au, As, Fe, Sb and Pb were deposited in large white quartz veins, then subsequently concentrated into irregular vein-parallel stylolites. It is inferred that this initial fluid has been produced through metamorphic dehydration reactions at depth. Subsequently an influx of Cu, Mo and Pb overprinted the initial mineralisation during deformation. This fluid may have been generated throughout the second phase of metamorphism known from the Greenland Group. Shallow Cu-Mo-bearing porphyry intrusions known from the area could possibly provide a plausible source candidate for these base metals. Following this, supergene enrichment of mineralised greywacke within the alteration halo may have occurred, and locally enriched the Au. In summary, the Big River South – St George deposits both host significant gold mineralisation; however, the sporadic occurrence of gold means the mines are unlikely to contain the high Au grades of other locations in the goldfield.

112p some A3

2015Shand

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